Electrophotography (laser printing) is a technology often used in printers which are connected to computer systems. Laser printers are popular because of their fast image printing speed. Comparatively, laser printers print images much faster than ink jet printers. However, the color quality of color images printed by laser printers using dry toner is not as high as the color quality of color images printed by ink jet printers.
The color quality of laser printers suffers because of the physical characteristics of the dry toner used in laser printers. The color quality is limited in part by the useable size (greater than 7 .mu.m in diameter) of the particles of the dry toner. Dry particles much smaller than 7 .mu.m in diameter are preferred for color, but tend to act like dust or smoke particles which do not settle if disturbed. The unsettled particles are generally considered to be a health hazard. Furthermore, dry toner particles do not work well as light filters because of the relative size of the particles compared to the wavelength of visible light.
Attempts have been made to reduce the useable size of the dry toner particles by suspending the particles within a liquid toner bath. Liquid electrophotography (LEP) uses toner particles less than 0.1 micrometers in size and an electrophoretic process to transport the particles through the liquid. Electrophoresis requires the liquid to be non-conductive and have a low viscosity. Generally, the optimal liquids for particle suspension tend to be hydrocarbon oils. Hydrocarbon oils, however, have some basic useability problems. The liquid oils can be prone to spillage, can emit undesirable vapors into the air, and under certain circumstances are flammable. The liquid toners must have a low viscosity for high toner mobility. Therefore, the toners usually have a very low solid colorant content of about 2% by volume.
An advantage of LEP is that liquid toner inks contain very small pigment particles. The size of the particles is very close to the wavelengths of the light being filtered by the particles. The particles act as semi-transparent light filters and subtract light of certain wavelengths from the otherwise white light reflecting from white paper on which the ink is deposited. In comparison, the large particles in dry toner scatter light because of the large physical size of the particles compared to the wavelengths of incident light. Furthermore, larger particles reflect light at air voids formed between multiple toner particles. The reflected light is uncolored by the particles and appears as "white" light which de-saturates the intended color. In contrast, liquid inks with pigment particles less than 0.1 micrometers in diameter can provide high color strength and color quality. The pigment particles generally include the primary colors of cyan, yellow, magenta and black. These primary colors can be mixed to accurately produce all other visible colors. In contrast, the scattered light resulting from dry toner desaturates and lowers the luminance value of the colors printed. This gives the dry toner prints a smaller color gamut.
Ink jet printers offer certain advantages over LEP printers. Ink jet printers hold ink in foam sponges inside pens that are not prone to spillage. The liquid solvent used for the inks is water. Water is not flammable and is not a source of undesirable vapors. Therefore, the release of water into the room air surrounding an ink jet printer is not as great an issue as when hydrocarbon oils are released into the air surrounding an LEP printer.
Many ink jet inks, however, contain dyes instead of pigments. Dyes are similar to pigments but are composed of free molecules of colorant dissolved in a carrier fluid. The dye molecules are smaller than the approximately 0.1 .mu.m diameter pigment particles and provide good color strength. Dye molecules, however, are susceptible to fading after prolonged exposure to light. Pigment particles are more colorfast but can cause nozzle orifices to clog in ink jet applications.
Ink jet printers usually print much slower than laser printers. Ink jet printers transport ink to a print medium through nozzles which are connected to chambers which hold the ink. Ink jet ink comprises mostly water to facilitate formation of an expanding steam bubble which ejects an ink drop. Usually, the colorant concentration of the ink is limited to about 2%-5%. Therefore, the printing operation transfers a relatively large amount of water to the print medium. Above a certain printing speed, water is applied faster than the print medium can absorb it. As a result, the ink will puddle. The puddles can run together, reducing the definition of the printed images and blending the ink, forming incorrect colors. Paper fibers also swell due to absorbed water causing a distortion known as cockle which detracts from the appearance of the finished print.
U.S. Pat. No. 5,108,867 proposes another method of depositing ink on a print medium. The print medium in which an image is to be formed is coated with a uniform layer of a dry developer chemical. The print medium is then uniformly coated with a mix of three different types of microencapsulated photohardenable toner particles. The three different photohardenable toner particles each contain one of three colors of clear liquid leuco dyes, each color within a different microcapsule shell. Each of the three different photohardenable toner leuco dyes harden when exposed to light of a certain wavelength. An image is created by imagewise hardening select photohardenable toner particles and then crushing all the toner particles on the print medium releasing the leuco dyes within the unhardened toner particles onto the print medium. The leuco dyes from unhardened capsules mix with the developer on the print medium and form a primary color. The dyes from hardened capsules cannot mix with the developer and have no color effect. For example, if the color red is to be formed on a region of a print medium, the medium is exposed to red light. The red light causes the photohardenable capsules containing the leuco dye for the color yellow to harden. The unhardened photohardenable toner particles containing cyan and magenta leuco dyes are crushed which releases the dyes onto the print medium. The resulting cyan and magenta colors mix forming a region of red on the print medium. An undesirable feature of this method is that the medium must be uniformly coated with the developer chemical and substantially covered with a mixture of all three photohardenable toner particles rather than only depositing the photo hardenable particles where ink is to be deposited on the medium. Another undesirable feature is that a plurality of light sources are required to provide light having the wavelengths required to harden the desired photohardenable toner particles.
Existing electrophotographic printer technologies require a heat fuser for bonding toner to a print medium after the toner has been deposited on the print medium. Heat fusers are undesirable for several reasons. Firstly, heat fusers dissipate energy. Many heat fusers dissipate energy the entire time that the associated printer is powered even if the printer is not actively printing. Secondly, heat fusers require a warm-up period before the heat fuser can be used. Therefore, a period of time is required between the point in time that the printer is turned on and the point in time at which one can create a print. Thirdly, the heat fuser transfers energy to the print medium to dry the ink deposited on the print medium. Print speed can be limited by the rate at which heat can be transferred to the print medium.
U.S. Pat. Nos. 5,463,454 and 5,428,435 propose methods of printing images in which the heat fusers operate at lower temperatures than required by other types of printers. However, even at a reduced temperature, the same limitations associated with heat fusers still exist.
It would be desirable to have a toner, apparatus and method of printing which is quick and which prints high quality color images. The desired toner, apparatus and method would not suffer from the environmental and health concerns of the above mentioned methods. Furthermore, the desired apparatus and method would be simple, would not require a heat fuser, would only deposit ink where required to form the desired image, and would not require special coated paper.